1. The field of art to which the invention pertains comprises the art of measuring and testing of fluid pressures.
2. Ever since the invention of a pressure displaceable tube by E. Bourdon in U.S. Pat. No. 9,163, the Bourdon tube has by and large become the industry standard in pressure instruments of all types. Basically, the tube in flattened form is appropriately coiled and on exposure to pressure changes applied either internally or externally, responds predictably in a correlated winding or unwinding displacement at its free end. By translating tube displacement via a geared movement or the like into a useful output form, it is possible to derive various pressure sensitive instruments, the most common of which is the pressure gauge.
Fundamentally important in the fabrication of a Bourdon tube is a material composition of appropriate properties. Not only must the material composition have sufficient strength to withstand the pressures to which it is to be subjected but it must likewise by characterized by good ductility and weldability to enable desired shaping and pressure-tight assembly within the instrument in which it is to be used.
Depending on the contemplated service, it is common to construct such Bourdon tubes of a chemically compatible composition such as a copper or nickel base alloy or from various steel alloys commercially available. For corrosion service, a commonly used alloy is AISI type 316 stainless steel. While particularly suitable for a wide variety of corrosive environments AISI type 316 stainless steel is nonetheless susceptible to stress corrosion cracking in the presence of chlorides and is therefore generally unsuitable for direct use in the processing of crude oil or the like. In those instances where exposure to chlorides and sulfides is likely to be encountered, it has been common to employ a diaphragm seal of the type disclosed, for example, in U.S. Pat. No. 3,202,063. The diaphragm seal in those applications acts as a guard by being positioned intervening between the measured fluid and the Bourdon tube in order to protect the latter from chemical attack by the former. Needless to say, such diaphragm seals are an added expense to both install and maintain yet despite their added costs it has not heretofore been known how to effect their elimination for service conditions likely to expose the Bourdon tube to chlorides and sulfides in the measured fluid.